I wrote this article for my British car club newsletter last month. Please tag on with comments if I've missed something or you have a tip or trick I've missed. Please PM me if you'd like to re-post or reprint somewhere--I'm happy to help.
If your English bike or car has a speedometer or tachometer built in the late 1950s or later that just does not Ďmeter right, youíve probably either tried to pretend it was a transient problem that would go away as mysteriously as it appeared or looked into sending your instrument off for restoration. If youíve done the later, you probably recoiled at the shocking cost of having someone restore the instrument for you.
Iím not here to tell you that restoration services are a rip off. Theyíre not. The last quote I saw was $260 for a gauge restoration. That included new glass, bezels, rubber parts, new face, cleaning, painting, shimming, re-bushing, and calibration amongst other things. The end result IS fabulous for a concourse restoration. However, if you are cheap and/or not doing a concourse restoration, there is a lot you can accomplish at home with patience and a more palatable outlay of cash. The above assumes that your instrument has an acceptable face and that the internal mechanism is only dirty or maladjusted rather than worn out.
Some of the larger automotive gauges have locking bezels that are removable. If so, thatís great. Just twist off the bezel and protect the glass at all costs. For the motorcycle crowd, Smithís rolled the back lip of the bezel over to improve the instrumentís weather resistance and seemingly to ensure you couldnít get into the gauge ever. I donít know about you, but Iíve prided myself over the years on fixing items that have no user serviceable parts. Iíve opened up sealed relays and emissions valves to get them back into commission, so it was no surprise to find myself carefully prising up the rolled lip on my Ďsealedí BSA tachometer.
Iíve been inside my Jaguarís tach and speedo before, so I expected this was no different. It wasnít. Of course I broke the glass getting the bezel off, but the bezel kits come with new glass and seals in addition to the replacement bezel. I just ordered two kits to the tune of $40 each from Eurojamb.com
. They are nice kits and really the only big cash outlay for this project. I twisted a screwdriver for a while to get the bezel off. Caution, this is a great way to end up at the emergency room with a screwdriver through your hand. Take my advice, just grind the rolled edge off. Itís false economy to try and save the bezel. Once the bezel and glass are off, save the glass if it hasnít broken yet. You never know when a spare might be handy. Now remove the screws from the back and separate the core from the case, being very careful not to disturb the needle.
Relocate the project to your sterile bench, above your meticulously clean floor that will allow you to find the parts you are about to lose.
Underneath the gauge face, you will see a section of the frame and a silver pickup disk that moves when you move the needle. With the needle at rest against its peg, take a Sharpie and make a mark that crosses both the disk and the frame. This is the witness mark that will later let you correctly locate the needle when you reassemble the gauge. Note, this is much harder on a speedometer than a tach as the odometer mechanisms hinder access.
If able, round up an assistant to steady the gauge. Protect the gauge face with some tape, plastic, or heavy paper and place the tips of two blade screwdrivers under each side of the needleís center boss. Twist them equally to apply even pressure to both sides of the needle to remove (launch) the needle. After you and your assistant have recovered the needle, open some beers. The hard part is over.
Now itís time to remove the face, which is held on by rivets! Donít despair, take a closer look. These are actually reusable and removable. Take a jewelerís screwdriver or other small tool and push the small dowel all the way through the rivet. Recover the dowels and the rivets. Lift off the gauge face. There is a tiny wire poking through the face that the needle rests against at zero speed or RPM. It is actually the end of a spring welded to the back.
Pull this spring wire back through its hole and let it rest against the back of the face. Wash the face and needle with mild soap and water. After you have dried the parts, go over the center boss of the needle with metal polish and a rag. Use a good quality wax and rub down the gauge face. This cleans the face further and takes away any oxidation.
Before doing any further disassembly, locate the adjustor screw on the back and screw it in tight, counting how many half turns it takes to hit the stop. Write it down! When you reassemble the gauge, youíll use this number to restore the instrument to its pre-disassembly adjustment.
Go ahead and remove the top frame bar along with the adjuster lever. Unscrew the adjuster from the back, and then remove the top reusable rivets. The whole assembly of spring, shaft, top brass bar, adjustment lever, and pickup disk should all come away. Wiggle the needle shaft to check for excess play. If the needle shaft has a lot of side to side movement, your instrumentís bushing has expired. A worn bushing will likely cause the needle of your gauge to jitter after you reassemble it. If you need the gauge re-bushed and feel up to more advanced work, this link: http://bullfire.net/Triumph/Triumph34/Triumph34.html
features a great tutorial on soldering in a new bushing.
Now itís time for the final disassembly. Remove the cap from the bottom of the shaft. It just pulls off. If this cap is missing, youíve probably just solved the problem that led you here in the first place. Without the cap, the cable sheath tightens against the rotating shaft of the gauge, locking it in position. When the inner shaft tries to spin, it canít and shears. Find a right-sized long socket or punch and drift the central shaft out through the bottom of the gauge. Collect the shaft, worm gear, and magnetic disk.
Clean the parts, particularly paying attention to the bushing on the inside of the frame. Metal polish is a good choice for this job, just make sure to clean out all of the polish.
Lube the bushing and shaft with grease. Donít overdo it. Any excess will foul the inside of the gauge.
Drive the worm gear onto the shaft and check for end float.
You want as little play as possible without binding the free movement of the shaft. Drive on the magnetic disk as well and make sure you havenít eliminated the end float.
Replace the spring, shaft, top brass bar, adjustment lever, and pickup disk. Screw the adjuster in until it hits the stop, then back it out by the number of half turns you recorded during disassembly. Install the cleaned instrument face.
Turn the pickup disk to line up with the witness marks and carefully press on the needle such that it is touching the stop peg with the witness marks aligned.
Paint the case and open another beer.
Test functionality of your rebuilt core by hooking up a drill with a screwdriver tip sized to engage the square receptacle in the back of the instrument.
With the drill turning counterclockwise, you should see the gauge register. The odometer isnít adjustable, but the needleís rate is. For a ballpark calibration, it should take about 18 seconds to go a 10th of a mile at 20 mph. To calibrate a tach, youíll need to dig up an old dwell meter and use the RPM function. For a single ignition coil driving on a twin cylinder, multiply the dwell meterís RPM reading for an eight cylinder engine by four to get an accurate motorcycle RPM.
From your bezel kit, install the replacement rubber and screw the core back into the case. Clean the inside of the glass and place the glass and bezel on the instrument. If you have a big automotive gauge, twist the bezel into place and be done with the job. For the motorcycle crowd, place the bezel against a piece of soft wood and compress gently in a vice. If you somehow figure out how to roll the back edge of the bezel without the special factory tool, thatís great.
For me, my BSA gauges go into the big rubber bosses, so no one will ever see my handiwork. I will take a punch and bend over the bezel in about eight places. No, this wonít seal out the weather, but I can always add silicone sealant. What this approach does get me is a second chance to ever get back inside if I need to.
Congratulations, you did it yourself and it is a thing of beauty! Redshirt